This invention relates to weatherable multilayer resinous articles and their preparation. More particularly, it relates to articles in sheet form having a protective arylate polymer coating.
Various polymeric articles have a problem of long term color instability. This causes yellowing of the polymer, detracting from its transparency and attractiveness. Loss of gloss can also be an undesirable long term phenomenon.
Yellowing of polymers is often caused by the action of ultraviolet radiation, which is why such yellowing is frequently designated “photoyellowing”. Numerous means for suppressing photoyellowing have been employed and proposed. Many of these involve incorporation in the polymer of ultraviolet absorbing compounds (UVA's). For the most part, UVA's are low molecular weight compounds and they must be employed at relatively low levels, typically up to 1% by weight, to avoid degradation of the physical properties of the polymer such as impact strength and high temperature properties as reflected in heat distortion temperature. Such levels may be inadequate to afford sufficient protection. Another problem of concern with polymers such as aromatic polycarbonates and addition polymers of alkenylaromatic compounds such as styrene is susceptibility to attack by organic liquids.
One way of protecting a resinous article against photoyellowing and loss of gloss is to apply a coating of a weatherable second polymer, the term “weatherable” as used herein signifying resistance to such phenomena. Weatherable polymers suitable for this purpose include resorcinol isophthalate/terephthalate copolyarylates. This is the subject of Cohen et al., J Poly. Sci., Part A-1, 9, 3263-3299 (1971), and certain related US patents of Monsanto Company including U.S. Pat. Nos. 3,444,129, 3,460,961, 3,492,261 and 3,503,779.
The only method disclosed therein for the application of the weatherable polymer, however, is by solution coating followed by evaporation of the solvent. This method has numerous deficiencies, some of which are mentioned in the Cohen et al. paper at page 3267: namely, the necessity to use high priced and toxic solvents, the inherently low concentration of the arylate polymer in the solvent and the tendency of the solutions to gel. Accordingly, the described copolyarylates were considered “unacceptable coating candidates”.
Other serious deficiencies of solvent coating have been discovered since the publication of the Cohen et al. paper and related patents. A principal one is an inherent property of the use of a solvent: it cannot possibly be completely removed by any procedure occurring at a level below the glass transition temperature of the substrate, which is impractical since it causes physical distortion.
Therefore, the solvent remains present in sufficient quantities to adversely affect the properties of the substrate polymer. This can occur by way of volatilization of the remaining traces of solvent during subsequent heat treatment such as thermoforming, lamination, in-mold decoration or baking in a paint oven. On a cosmetic level, the results can include blistering, bubbling, cracking and void formations within the substrate and coating, degrading the appearance of the resulting article. Other adverse results can be degradation of physical properties by crazing, cracking and embrittlement of the substrate polymer.
Since the publication of the Cohen et al. paper, the industry has further recognized the desirability of recycling resinous articles by regrinding, to minimize the deposition of non-biodegradable waste in landfills. Many multilayer resinous articles cannot be recycled, since the substrate and the coating are often incompatible with each other and the recycle operation, which includes conversion to a blend of the two polymers, produces a material with inferior physical properties.
Japanese Kokai 1/199,841 discloses articles having a substrate layer comprising at least 90 mole percent poly(ethylene terephthalate) and a gas barrier coating layer which is a polyester of resorcinol and isophthalic acid, optionally with copolyester units derived from another dicarboxylic acid such as terephthalic acid, naphthalenedicarboxylic acid or various other specifically named dicarboxylic acids. The disclosed articles may be prepared by a series of operations including co-injection molding which are essentially performed entirely in the melt, thereby overcoming the aforementioned deficiencies of solution coating. However, the only types of articles disclosed are bottles, which are produced from a co-injection molded parison by subsequent blow molding. Larger articles intended for outdoor use, such as external automobile body parts, are not disclosed and no method for their production is suggested, nor are articles in which the substrate layer is anything other than poly(ethylene terephthalate).
It remains of interest, therefore, to develop a method for preparing weatherable, solvent resistant multilayer articles which are capable of use for such varied purposes as body parts for outdoor vehicles and devices such as automobiles, and which can be prepared without adverse solvent effects. It is further of interest to prepare articles which include only mutually compatible polymers, making them suitable for recycle.